Field of the Invention (Technical Field)
The present invention relates to a method for covering particles having a diameter of maximally 60 μm by means of atomic layer deposition. The present invention also relates to particles obtainable by such method, and a battery containing said particles.
Description of Related Art
Such a method is known from the art. Although hereinafter mainly reference will be made to particles to be used in a battery, for example and preferably lithium containing particles, such as LiMn2O4, LiCoO2 or LiNiO2 as well as other lithium containing materials, such as LiFePO4 and others, the method can be used for subjecting all kinds of particles in the said size range by means of atomic layer deposition.
In the art, the use of lithium ion batteries has many advantages over other cathode material containing batteries, especially with respect to rechargeable batteries. Compared to nickel-cadmium batteries and nickel-metal-hydride batteries, the output voltage of lithium ion batteries is higher. Secondly, the energy density is higher, resulting in smaller and lighter batteries. Other advantages of lithium ion batteries are a low self-discharge, good cycle-life and very low maintenance. Drawbacks of lithium ion materials are the relatively high costs and long charging times, and the fact that the batteries age in time, whether they are being used or not.
During the discharge of the lithium ion batteries, lithium ions are transferred from the negative electrode side of the battery to the positive electrode side. Recent research activities have provided new electrode materials, that provide an improved transport of lithium ions. An example of this material is Li4Ti5O12, which is used as a negative electrode material having the spinel structure. This material has a three-dimensional structure for lithium intercalation (the insertion of lithium into the crystal lattice). With this material, high charge and discharge rates are possible. A draw-back of this material is that the potential at which lithium intercalation occurs is much higher than that for negative electrode materials used thus far. As a result, the battery will have a lower output voltage than was usual for lithium ion batteries. To compensate for this problem, new positive electrode materials have been developed with higher potentials than the currently used materials. Potential (Possible) new positive electrode materials are based on LiMn2O4 (comprising a 50/50 combination of Mn3+ and Mn4+), with possible additives like Mg, Ni, like LiMgxNi0.5-xMn1.5O4, (comprising only Mn4+) which is also of the spinel-type. The positive electrode voltage is 4.7-4.9 V, against Li/Li+. Therefore, the battery output voltage for a combination consisting of Li4Ti5O12/LiMgxNi0.5-xMn1.5O4 (comprising Mn4+, and a combination of Ni2+ and Ni4+) can be 3.2-3.4 V, which still is a very acceptable value.
Hereafter in this description the negative electrode will be referred to (identified) as the anode and the positive electrode will be referred to (identified) as the cathode.
A problem with the above identified cathode material is the dissolution of transition metal ions, especially Mn-ions, in the electrolyte. When this occurs, the structure of the material changes and there is a smaller number of positions available for lithium intercalation. In addition, the high oxidation ability of Mn4+-ions may lead to a decomposition of the solvents in the electrolyte. These factors lead to a capacity loss that is independent of the cycling but proceeds progressively in time. The capacity fading increases with temperature: when Li-ion batteries are stored at temperatures of 60° C., a battery may lose up to 40% of its capacity in only three months time. The problem is more severe for high-voltage materials (e.g. Mn and Fe comprising materials) than for “regular” cathode materials. A specific example of a Fe-containing cathode material, is LiFexTiyMn2-x-yO4 wherein 0<y<0.3.
Recently, also research has been performed dedicated to the use of nano-powders in batteries. These powders have several advantages over the current cathode or anode materials. Firstly, the surface area per weight increases strongly, leading to enhanced charge transfer (faster charging). Secondly, the diffusion lengths for Li-ions are very short, which enhances the power performance by increasing the effective capacity for lithium storage. Thirdly, the nano-powders are much more resistant to stresses due to expansion and shrinking during intercalation and de-intercalation of the lithium ions, which cause crystal fatigue and loss of capacity in regular cathode materials.
An important drawback of nano-materials in batteries is the increased dissolution of the transition metal ions. This dissolution in the electrolyte is a surface related problem, and therefore increases very fast with decreasing particle size.